The older Irish grid reference system required GPS measurements to be "translated". The more precise the GPS measurements were, the more the translation process introduced inaccuracies. While the existing UTMco-ordinate system partly fulfilled the requirement for direct GPS compatibility it had some drawbacks, including varying levels of distortion across the island due to the central meridian being at the west coast of Ireland. The new system needed to satisfy various criteria: GPS compatibility, map distortion for the whole island of Ireland had to be minimal, it was to be conformal and backward compatible with existing mapping. A customised Transverse Mercator projection was chosen. ITM and the older more established Irish Grid will be used in parallel. As a result ITM coordinates had to be obviously different so users would not confuse the two. This was done by shifting the ITM false origin further into the Atlantic and thereby creating substantially different co-ordinate numbers for any given location. While OSi and OSNI intend to supply map information in the older Irish Grid format into the future, the Irish Institution of Surveyors has recommended that ITM be adopted as soon as possible as the preferred official co-ordinate system for Ireland.
Examples
An ITM co-ordinate is generally given as a pair of two six-digit numbers. The first number is always the easting and the second is the northing. The easting and northing are in metres from the false origin. The ITM co-ordinate for the Spire of Dublin on O'Connell Street is: The first figure is the easting and means that the location is 715,830 metres east from the false origin. The second figure is the northing and puts the location 734,697 metres north of the false origin The equivalent Irish Grid co-ordinate for the same location is: The Spire of Dublin example provides a fix for a location that is accurate to 1 metre. With ITM it is possible to give a more accurate co-ordinate for a given location by using a decimal point after the initial six figure easting and northing. The ITM co-ordinate for the passive GPS station at the OSi office is: This ITM co-ordinate has three digits behind the decimal point which gives a fix for a location with millimetre accuracy. Also notice how the easting in this example is indicated with an “E” and likewise an “N” for the northing. The fact that the co-ordinate is in metres is indicated by the lowercase m. With ITM there is no provision for using myriad letters and truncated coordinates as there is with the Irish Grid. Every co-ordinate must be given with at least a six-digit easting and northing from the false origin.
